The 5-String Banjo, played in the 3-finger "Scruggs" style is one of the defining sounds of bluegrass music. Banjos have a long history and have gone through many changes since first coming to the Americas on slave ships from West Africa. Because of this, it is important to make sure you're playing the appropriate banjo for each style of banjo music.
Left - Gretsch G9400 Broadkaster Deluxe - This 5-string resonator banjo is what most people think of when discussing bluegrass banjos. It has 5 strings - including the short 5th string that gives it the distinctive sound, a resonator on the back - to project loudly in a bluegrass ensemble, and a flathead tone ring under the head for a increased depth of sound. This banjo is normally played with a thumb and two finger picks in order to get the characteristic attack of the bluegrass banjo.
Middle - Gretsch G9451 Dixie Deluxe - This 5-string open-back banjo should not be confused with its bluegrass sibling. Although it still has 5 strings, it is usually played with bare fingers in a clawhammer or frailing style - often heard in old time music. Lacking a resonator, it doesn't project as much and has a much more "round" sound to it.
Right - Stromberg-Voisinet Tenor Banjo - Notice that this banjo is missing the 5th string that is characteristic of bluegrass and old time banjos. They are most commonly played with a pick, similar to guitar. Tenor banjos find a ready home in jazz, jugband, and celtic music.